It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone", the presence of the marker is detected and an alarm is generated. Some markers are intended to be removed at the checkout counter upon payment for the merchandise. Other types of markers remain attached to the merchandise but are deactivated upon checkout by a deactivation device which changes a characteristic of the marker so that the marker will no longer be detectable at the interrogation zone.
A known type of EAS system employs markers which include an LC resonant circuit. An example of such a system is disclosed in U.S. Pat. No. 3,810,147. The circuit is typically formed on a substrate by printed or etched circuit techniques and includes a conductive path to form a coil on one side of the substrate. The coil is connected to a capacitor formed of capacitor plates that are on opposite sides of the substrate. The resonant circuit of the marker is tuned to a predetermined frequency. The detection equipment of the EAS system includes a transmitter which radiates an interrogation signal in the interrogation zone. The interrogation signal is swept through a frequency range which includes the predetermined tuning frequency of the marker. When an active marker is present in the interrogation zone, receiving equipment at the zone detects a change in the interrogation field at the tuned frequency because of the resonance of the resonant circuit of the marker.
It is known to provide a resonant circuit marker that can be deactivated by including in the marker circuitry a fusible link. The fusible link can be caused to fuse upon being energized by application of an electromagnetic field at a predetermined frequency, which may be the resonant frequency of the marker circuit itself, or the resonant frequency of a deactivation circuit associated with the fusible link. When the fusible link is energized and caused to fuse, an open circuit is formed in the resonant circuit of the marker, causing the marker to be detuned and no longer detectable by the detection portion of the EAS system.
As an alternative technique for deactivating resonant circuit markers, the dielectric between the capacitor plates may be broken down by application of a high energy pulse at the marker's tuned frequency. It is known, for example, to provide dimples in one of the capacitor plates, or to provide other structure which facilitates formation of a breakdown path between the capacitor plates.
Some improvements in known resonant circuit EAS markers are desirable. For example, it would be worthwhile to increase the reliability with which markers of this type can be detected. Further, it would be desirable to provide a marker that can be detected without using a swept-frequency interrogation transmitter. Furthermore, known techniques for deactivating resonant circuit markers are irreversible, in that once a fusible link is fused or the capacitor is broken down, the marker cannot be reactivated. It would be useful to provide a resonant circuit EAS marker that can be restored to an active condition after the marker has been deactivated.